Desired electrical properties of steels used for making motor laminations are low core loss and high permeability. Those steels which are stress relief annealed after punching also should have properties which minimize distortion, warpage and delamination during the annealing of the lamination stacks.
Continuously annealed, silicon steels are conventionally used for motors, transformers, generators and similar electrical products. Continuously annealed silicon steels can be processed by techniques well known in the art to obtain low core loss and high permeability. Since these steels are substantially free of strain, they can be used in the as-punched condition (in which the steel as sold is commonly referred to as fully processed) or if better magnetic properties are desired the steel can be finally annealed by the electrical apparatus manufacturer after punching of the laminations (in which case the steel as sold is commonly referred to as semi-processed) with little danger of delamination, warpage, or distortion. A disadvantage of this practice is that the electrical steel sheet manufacturer is required to have a continuous annealing facility.
In order to avoid a continuous annealing operation, practices have been developed to produce cold rolled motor lamination steel by standard cold rolled sheet processing including batch annealing followed by temper rolling. In order to obtain the desired magnetic properties of high permeability and low core loss, it is common to temper roll the steel with a heavy reduction in thickness on the order of 7%. Electrical steels processed by batch annealing and heavy temper rolling followed by a final stress relief anneal after the punching operations develop acceptable core loss and permeability.
Fully-processed electrical steels are used by customers in the as-punched/stamped condition without a subsequent annealing operation being required. Standard cold-rolled electrical steels are unsuitable for most fully-processed applications due to strain remaining in the material. Fully processed materials are produced utilizing continuous anneal lines since no additional strain is required to provide acceptable flatness. Batch annealed materials, however, do not have acceptable flatness and require some strain simply to provide a flat product. This strain is usually provided by conventional temper rolling.
Conventional hot rolling practices for cold rolled motor lamination electrical steels use high finishing temperatures in the austenite region. While a hot band annealing step may be omitted, high coiling temperatures of, for example, about 1400.degree. F., are used to promote "self annealing" of the generated hot bands. This process is believed to produce optimal magnetic properties. Processes employing austenite hot roll finishing temperatures result in poorer magnetic properties, particularly permeability, as coiling temperatures are decreased. It is believed that in such processes coiling should be carried out at temperatures of at least 1200.degree. F. to avoid degradation of magnetic properties.
Hot band annealing is used in such methods to improve magnetic properties of the steel. However, despite any improvements in magnetic properties, the hot band annealing process is undesirable in that it is an extra step, expensive equipment for annealing at relatively high temperatures is required, and the hot band anneal process lasts several days if batch type facilities are used. As a result, the hot band annealing step increases the cost of the steel.
Cleanliness of the steel strip is an increasing concern of some customers of motor lamination steel. Fine iron particles on the surface of the strip can create problems for some customers. One problem is that the iron fines may come off the strip and build up in roller leveling equipment used to remove coil set. This requires cleaning the equipment.
Another problem occurs during stamping. Indexing rolls cause the strip to be fed precise distances into dies for successive punching of shapes. The distance the strip is indexed is determined by the arc of the indexing rolls. The iron fines may adhere to the indexing rolls and thus, change their diameter. This changes the feed length and causes the strip to be indexed by an improper amount, which can require the process to be stopped for cleaning of the indexing rolls. Yet another problem is that the dies may require cleaning when a build up on the dies prevents proper flow of the material. Of course, stopping the process is undesirable in that it decreases the productivity of stamping and results in the expense of cleaning the equipment.